1. Field of the Invention
This invention relates to a telephone signal detecting circuit, which can distinguish a line reversal signal and a ring signal.
2. Description of Prior Art
A telephone system is either on-hook or off-hook. When a telephone system is on-hook, there are different signaling sequences to interrupt such state for different standards. Referring to FIG. 1, which illustrates the timing diagram of Caller ID signals. FIG. 1a represents the timing of Caller ID for BT""s (British Telecom) standard. That is, a Caller ID signal is sent to the telephone system before a ring signal. However, the telephone system stays in a standby state, which is very low-power consumption when it is on-hook. Therefore a line reversal signal is used to wake up the microprocessor of the telephone system before the arrival of the Caller ID signal. FIG. 1b shows the Bellcore""s standard, caller ID signal is arrived after the first ringing. However, the caller ID signal is not to be received when the line reversal signal appears. A telephone set, which can identify the calling party, normally determines whether a received signal is a line reversal signal or a ring signal according to the period of interrupting the microprocessor of the telephone set, and then determines whether to activate the caller identifying IC. Another simple method is to activate the IC no matter whether the microprocessor is interrupted by a line reversal signal or a ring signal. After passing a period of time, the caller identifying IC is turned off if no caller ID signal is received. However, the power is wasted no matter which method is adopted. Moreover, the first method also wastes the time of the microprocessor in the telephone set.
FIG. 2 shows a diagram for the detection circuit for the line reversal signal or the ring signal in a normal telephone set. In FIG. 2 there are only illustrated a rectifier circuit 10 and a ring detector 20 since it is used to explain the detection for the signal input of the telephone set. In the rectifier circuit 10, the capacitors C1, C2 can block the DC voltage in the tip/ring line. The AC signal can be obtained by the resistor R1 and capacitor C1 or the resistor R2 and the capacitor C2 and then rectified by the rectifier consisting of D1, D2, D3 and D4. The rectified signal is input to the ring detector 20. A part of the signal is sent to the input terminal RNGDI (ring detector input) of the Schmitt trigger 201 through the resistors R3 and R4. Therefore, if the change of the signal in the tip/ring line is large enough, Q1 will discharge the capacitor C3, and the output terminal RNGON becomes low level. When the signal in the tip/ring line is stable, Q1 will not discharge the capacitor C3. The capacitor C3 is then charged by VCC through the resistor R5, and the output terminal RNGON becomes high level. FIG. 3 illustrates a diagram of the waveform for the signal on RNGRC and RNGON while inputting the line reversal signal or the ring signal. The output terminal RNGON is low level when the voltage of RNGRC drops off. Normally speaking, it can be identified whether there is a line reversal signal or a ring signal in accordance with the voltage level of the output terminal RNGON.
However, a bouncing phenomenon often occurs when the exchange activates a line reversal signal or a ring signal, therefore the transistor Q1 is in a status of xe2x80x9con, off, on, offxe2x80x9d at the beginning of discharging the capacitor C3, as shown in FIG. 4. A series of rising and falling of the signal similar to a ripple causes the error of signal detection.
The object of this invention is to provide a telephone signal detecting circuit, which can identify an input signal as a line reversal signal or a ring signal before the end of the detection for the line reversal signal or the ring signal.
The telephone signal detection according to this invention is to latch the identifying signal before the end of the detection for the line reversal signal or the ring signal and interrupt the microprocessor. The microprocessor recognizes whether the input signal is a line reversal signal or a ring signal according to the identifying signal. Then the interrupt signal is clear, and the microprocessor can go on to the next task.
In addition, the design of the circuit of this invention uses Schmitt triggers of different trigger levels, wherein the Schmitt triggers having a higher trigger level are used to detect the input line reversal signal and ring signal, the Schmitt triggers having a lower trigger level are used to detect the ripple signals of the capacitor while detecting the ringing, and accordingly determine the ring signal. The ripple detecting circuit can overcome the signal detecting error caused by the bouncing effect occurred when the exchange activates the line reversal signal and the ring signal.
The telephone signal detecting circuit of this invention includes Schmitt triggers of two different trigger levels, which are a first level and a second level. As the capacitor is charged and discharged, the trigger having the first trigger level outputs a ring detecting signal when the voltage of the capacitor is lower than the first level, and the trigger having the second trigger level outputs a pulse signal corresponding to the ripple signal when the voltage of the capacitor is lower than the second trigger level. Furthermore, the ripple detecting circuit can filter out the bouncing noise by the driving of the trigger having the first trigger level and under the control of the trigger having the second trigger level, and outputs a ring signal after receiving the pulse signal. Thus the input signal is determined to be a line reversal signal when the ring detecting signal is low level and no ripple signal outputs. Otherwise the input signal is determined to be a ring signal if the ring detecting signal is low level and a ripple signal is output.